There are a variety of methods known for detecting the occurrence of fires. By way of example, one of the simplest methods entails discriminating the occurrence of a fire on the basis of a sensor level, i.e. detection data of a fire detector, wherein a fire signal is output when the sensor level exceeds a certain predetermined level. In this case, although the fire signal is output from the fire detector depending on whether or not the detection signal exceeds the predetermined level, various environmental conditions are not fully taken into account. In this connection, it is conceivable that the environmental information may be collected in addition to the detected information from the fire detector, wherein the fire discrimination is synthetically made on the basis of both the detection information and the environmental information. However, the environmental conditions are intrinsically ambiguous and consideration thereof in the fire discrimination can not always assure a sufficiently high reliability of a correct generation of the fire signal. In actuality, there frequently arise situations in which a phenomenon other than a fire triggers generation of a fire signal.